To support a payload while submerged, all underwater vehicles require buoyancy that is either provided by the pressure hull, floatation units attached to the hull, or both. Flotation units for manned deep submergence vehicles and remote autonomous (ROV) or autonomous underwater vehicle (AUV) systems must be capable of withstanding, in some cases, pressures at depths of 20,000 feet or more. In the past, flotation units for deep submersibles have been made from glass (a low elastic modulus material), steel, or ceramic spheres embedded in syntactic foam. The syntactic foam itself is a composite of plastic matrix such as epoxy and glass micro-spheres, which are typically micro-sized (e.g. the size of dust or other small particles).
The buoyancy of the syntactic foam is a function of the wall thickness of the glass micro-spheres and their packing density in the plastic matrix. The pressure resistance of the syntactic foam can be tailored by screening the glass micro-spheres for size and separation by density (wall thickness).
Syntactic foams have been developed for a wide range of ocean depths. The factor that limits their buoyancy is the packing density of the micro-spheres in the plastic matrix, which itself provides little, if any, buoyancy. By minimizing the volume of plastic matrix, the buoyancy of syntactic foam can be increased. This can be achieved by embedding relatively large glass or ceramic spheres with higher buoyancy than the foam itself. Larger spheres, hereinafter referred to as macro-spheres, provide more buoyancy than an equivalent volume of syntactic foam since the macro-spheres are not burdened with plastic matrix. Macro-spheres are typically an order of magnitude or two larger than micro-spheres (e.g., on the size of diameters in the inches or more). In the past, glass or ceramic macro-spheres have also been held in place in a framework made of plastic that is lighter than water.
Heretofore flotation units made of glass or ceramic macro-spheres embedded in syntactic foam have suffered from the problem that the macro-spheres have failed under pressures substantially lower than the pressures they can withstand when not embedded in the syntactic foam. Attempts to solve this problem by floating the macro-spheres in individual water filled chambers formed in the syntactic foam have been successful, but this approach involves an expensive fabrication process, and reduces the packing efficiency of the macro-spheres.
Accordingly, there is a need in the art to address the above and other-described problems.